Engine Balancing

So i got the unorthodox PS/AC eliminating underdrive crank pulley, and the guy at the machine shop says i need to have the engine spun balanced since the crank pulley is "part of the engines balancing". I thought they balanced the engine by adding weights to the flywheel, but he indicated that the holes that circumferate the crank pulley have something to do with the engine balancing. If this is the case then how can they call this underdrive pulley a "bolt on performance part" if you gotta rebalance ur engine to make it work. If anyone knows whether i actually need to spin balance my engine to put this pulley on, please enlighten me, because i would rather not as it is quite expensive. Thanks.

It is part of the rotating assembly and IIRC the unothrodox pulley does not have a harmonic balancer so if it isn't balanced properly it will vibrate like crazy. It might be balanced from the factory, but I wouldn't trust it. IMO you should have it all balanced.